Individuals who have sustained a traumatic brain injury (TBI) are at a higher risk for substance use disorders, and while clinicians and researchers widely acknowledge issues with opioid abuse and dependence, pre- clinical investigations have published data suggesting positive benefits of opioids as a potential therapeutic strategy for patients of TBI. However, concerns regarding the effect of opioid exposure on neuronal degeneration have also emerged in both pre-clinical and clinical settings. For example, lesions in white matter have been documented for methadone, morphine and oxycodone over-dosed patients. We acknowledge that the neurobiology of opioid use following TBI is difficult to study in clinical settings, especially in cases where pre-injury assessment may not be possible or accurate. We, therefore, propose to investigate the long-term effects of opioid use in our well-characterized animal model of repetitive mTBI (r-mTBI), thereby removing the biases inherent to human studies. As there are confounding data on the influence of opioid treatment or abuse on chronic outcomes after r-mTBI we hypothesize that investigation of the chronic effects of opioid treatment in our translational model of r-mTBI will identify any negative or positive effects on neurobehavior and neuropathology and help guide future, non-addictive, treatment interventions. The overarching aim of the proposed study is to investigate and refine our understanding of the chronic effects of two opioid agonists (Oxycodone, Methadone), and a non-selective opiate antagonist (Naloxone). In addition, this project will investigate the interaction between microglia and opioids; yet the role of microglia in the context of long-term opioid treatment and after brain injuries remains underexplored within the literature. In addition to pain relief, understanding the role of microglia after chronic opioid exposure might also be a way to prevent opioid tolerance and explain opioid induced hyperalgesia. In the first aim, male animals will be exposed to five mTBIs or five sham anesthesia (controls), and then treated with one of the two proposed opioid agonists, an opioid antagonist, or saline for a period of 6 months starting 24h post-last injury. The neurobehavioral performance will then be evaluated at both 1- and 6-months post-injury. In the second aim, neuropathological and biochemical analyses will be evaluated at 6 months post-injury. For both aims, we will evaluate the same outcome measures in injured versus control animals exposed to each pharmacological agent. We believe these findings will have broad applicability in both TBI and opioid research, as the data generated in this study will further the understanding of the complex interaction between the chronic exposure of opioid agonists and antagonist, TBI and the microglia cell population. By assessing nuanced aspects of neurobehavioral and pathological deficits, we will provide a framework from which informed decisions can then be made about the cellular and molecular mechanisms that are most important to target to reduce TBI-related pathology, and furthermore, which therapeutic intervention strategy best suits the patient. If any benefits are found with addictive opioids, then this will provide the impetus for development of non-addictive treatments targeting the same pathways. Within 18 months from the start date of this project, we will be able to determine: 1) Which opioids if any, provide the best neurological recovery based on the behavioral and neuropathological outcome markers; and 2) the role of microglia in response to chronic exposure to opiates by studying their morphological states.